Function hooking for OSX and Linux

function hooking for osx and linux

joe damato@joedamato

timetobleed.com

slides are ontimetobleed.com

(free jmpesp)

i’m not a security researcher.

call me a script kiddie: @joedamato

laughinglarry.com

slayerinc.com

dbgrady.files.wordpress.com

assembly is in att syntax

WTF is an ABI ?

WTF is an Application Binary

Interface ?

alignment

thomasgroup.com

calling convention

arianlim.wordpress.com

object file and library formats

tandemfs.org

hierarchy of specs

topatoco.com

System V ABI (271 pages)

System V ABI AMD64 Architecture Processor Supplement (128 pages)

System V ABI Intel386 Architecture Processor Supplement (377 pages)

MIPS, ARM, PPC, and IA-64 too!

mac osx x86-64 calling convention

based on

System V ABI AMD64 Architecture ! ! ! Processor Supplement

gregs-wines.com

alignment

thomasgroup.com

end of argument area must be aligned on a 16byte boundary.

and $0xfffffffffffffff0, %rsp

calling convention

arianlim.wordpress.com

• function arguments from left to right live in:%rdi, %rsi, %rdx, %rcx, %r8, %r9

• that’s for INTEGER class items.

• Other stuff gets passed on the stack (like on i386).

• registers are either caller or callee save

object file and library formats

tandemfs.org

steverubel.typepad.com

ELF Objects

en.wikipedia.org

ELF Objects• ELF objects have headers

• elf header (describes the elf object)

• program headers (describes segments)

• section headers (describes sections)

• libelf is useful for wandering the elf object extracting information.

• the executable and each .so has its own set of data

ELF Object sections• .text - code lives here

• .plt - stub code that helps to “resolve” absolute function addresses.

• .got.plt - absolute function addresses; used by .plt entries.

• .debug_info - debugging information

• .gnu_debuglink - checksum and filename for debug info

ELF Object sections

• .dynsym - maps exported symbol names to offsets

• .dynstr - stores exported symbol name strings

• .symtab - maps symbol names to offsets

• .strtab - symbol name strings

• more sections for other stuff.

vanachteren.net

Mach-O Objects

developer.apple.com

Mach-O Objects• Mach-O objects have load commands

• header (describes the mach-o object)

• load commands (describe layout and linkage info)

• segment commands (describes sections)

• dyld(3) describes some apis for touching mach-o objects

• the executable and each dylib/bundle has its own set of data

Mach-O sections

• __text - code lives here

• __symbol_stub1 - list of jmpq instructions for runtime dynamic linking

• __stub_helper - stub code that helps to “resolve” absolute function addresses.

• __la_symbol_ptr - absolute function addresses; used by symbol stub

Mach-O sections

• symtabs do not live in a segment, they have their own load commands.

• LC_SYMTAB - holds offsets for symbol table and string table.

• LC_DYSYMTAB - a list of 32bit offsets into LC_SYMTAB for dynamic symbols.

blog.makezine.com

nm

000000000048ac90 t Balloc

0000000000491270 T Init_Array

0000000000497520 T Init_Bignum

000000000041dc80 T Init_Binding

000000000049d9b0 T Init_Comparable

000000000049de30 T Init_Dir

00000000004a1080 T Init_Enumerable

00000000004a3720 T Init_Enumerator

00000000004a4f30 T Init_Exception

000000000042c2d0 T Init_File

0000000000434b90 T Init_GC

% nm /usr/bin/ruby

symbol “value”

symbol names

objdump% objdump -D /usr/bin/ruby

offsets opcodes instructions helpful metadata

readelf% readelf -a /usr/bin/ruby

This is a *tiny* subset of the data available

otool% otool -l /usr/bin/ruby

This is a *tiny* subset of the data available

nerve.com

strip• You can strip out whatever sections you

want....

• but your binary may not run.

• you need to leave the dynamic symbol/string tables intact or dynamic linking will not work.

bassfishin.com

Calling functions

callq *%rbx

callq 0xdeadbeef

other ways, too...

anatomy of a call412d16: e8 c1 36 02 00 callq 4363dc # <a_function>

412d1b: .....

address of this instruction

call opcode32bit displacement to the target function from the next instruction.

(objdump output)

anatomy of a call412d16: e8 c1 36 02 00 callq 4363dc # <a_function>

412d1b: .....

412d1b = 4363dc + 000236c1

(x86 is little endian)

(objdump output)

Hook a_function

Overwrite the displacement so that all calls to a_function actually call a different function instead.

It may look like this:int other_function() { /* do something good/bad */

/* be sure to call a_function! */ return a_function();}

codez are easy/* CHILL, it’s fucking psuedo code */

while (are_moar_bytes()) { curr_ins = next_ins; next_ins = get_next_ins(); if (curr_ins->type == INSN_CALL) { if ((hook_me - next_ins) == curr_ins->displacement) { /* found a call hook_me!*/ rewrite(curr_ins->displacement, (replacement_fn - next_ins)); return 0; } }}

... right?.....

lemur.com

32bit displacement• overwriting an existing call with another call

• stack will be aligned

• args are good to go

• can’t redirect to code that is outside of:

• [rip + 32bit displacement]

• you can scan the address space looking for an available page with mmap, though...

Doesn’t work for all

calling a function that is exported by a dynamic library works differently.

How runtime dynamic linking works (elf)

0x7ffff7afd6e6

.got.plt entryInitially, the .got.plt entry contains the address of the instruction after

the jmp.

How runtime dynamic linking works (elf)

0x7ffff7afd6e6

.got.plt entryAn ID is stored and the rtld is

invoked.

How runtime dynamic linking works (elf)

0x7ffff7b34ac0

.got.plt entryrtld writes the address of

rb_newobj to the .got.plt entry.

How runtime dynamic linking works (elf)

0x7ffff7b34ac0

.got.plt entryrtld writes the address of

rb_newobj to the .got.plt entry.

calls to the PLT entry jump immediately to rb_newobj now

that .got.plt is filled in.

rs.tacklewarehouse.com

Hook the GOT

Redirect execution by overwriting all the .got.plt entries for rb_newobj in each DSO with a handler function instead.

0xdeadbeef

.got.plt entryVALUE other_function() { new_obj = rb_newobj(); /* do something with new_obj */ return new_obj;}

Hook the GOT

NO, it isn’t. other_function() lives in it’s own DSO, so its calls to rb_newobj() use the .plt/.got.plt in its own DSO.

As long as we leave other_function()‘s DSO unmodified, we’ll avoid an infinite loop.

WAIT... other_function() calls rb_newobj() isn’t that an infinite loop?

vanachteren.net

tlaneve.files.wordpress.com

elf

mach-o

me

what else is left?

inline functions.

add_freelist• Can’t hook because add_freelist is inlined:

static inline voidadd_freelist(p) RVALUE *p;{ p->as.free.flags = 0; p->as.free.next = freelist; freelist = p;}

• The compiler has the option of inserting the instructions of this function directly into the callers.

• If this happens, you won’t see any calls.

So... what now?• Look carefully at the code:

static inline voidadd_freelist(p) RVALUE *p;{ p->as.free.flags = 0; p->as.free.next = freelist; freelist = p;}

• Notice that freelist gets updated.

• freelist has file level scope.

• hmmmm......

A (stupid) crazy idea• freelist has file level scope and lives at some

static address.

• add_freelist updates freelist, so...

• Why not search the binary for mov instructions that have freelist as the target!

• Overwrite that mov instruction with a call to our code!

• But... we have a problem.

• The system isn’t ready for a call instruction.

alignment

thomasgroup.com

calling convention

arianlim.wordpress.com

Isn’t ready? What?• The 64bit ABI says that the stack must be

aligned to a 16byte boundary after any/all arguments have been arranged.

• Since the overwrite is just some random mov, no way to guarantee that the stack is aligned.

• If we just plop in a call instruction, we won’t be able to arrange for arguments to get put in the right registers.

• So now what?

jmp

• Can use a jmp instruction.

• Transfer execution to an assembly stub generated at runtime.

• recreate the overwritten instruction

• set the system up to call a function

• do something good/bad

• jmp back when done to resume execution

picasaweb.google.com/lh/photo/-R3BPlqOq8MfQGFTduIqCA

checklist• save and restore caller/callee saved

registers.

• align the stack.

• recreate what was overwritten.

• arrange for any arguments your replacement function needs to end up in registers.

• invoke your code.

• resume execution as if nothing happened.

this instruction updates the freelist and comes from add_freelist:

Can’t overwrite it with a call instruction because the state of the system is not ready for a function call.

The jmp instruction and its offset are 5 bytes wide.Can’t grow or shrink the binary, so insert 2 one byte

NOPs.

shortened assembly stub

shortened assembly stub

shortened assembly stub

shortened assembly stub

shortened assembly stub

shortened assembly stub

shortened assembly stub

shortened assembly stub

void handler(VALUE freed_object) { mark_object_freed(freed_object); return;}

shortened assembly stub

and it actually works.

gem install memprofhttp://github.com/ice799/memprof

listverse.files.wordpress.com

Sample Outputrequire 'memprof'Memprof.startrequire "stringio"StringIO.newMemprof.stats

108 /custom/ree/lib/ruby/1.8/x86_64-linux/stringio.so:0:__node__ 14 test2.rb:3:String 2 /custom/ree/lib/ruby/1.8/x86_64-linux/stringio.so:0:Class 1 test2.rb:4:StringIO 1 test2.rb:4:String 1 test2.rb:3:Array 1 /custom/ree/lib/ruby/1.8/x86_64-linux/stringio.so:0:Enumerable

a web-based heap visualizer and leak analyzermemprof.com

memprof.coma web-based heap visualizer and leak analyzer

a web-based heap visualizer and leak analyzermemprof.com

memprof.coma web-based heap visualizer and leak analyzer

memprof.coma web-based heap visualizer and leak analyzer

memprof.coma web-based heap visualizer and leak analyzer

community.devexpress.com

config.middleware.use(Memprof::Tracer)

{ "time": 4.3442,

"rails": { "controller": "test", "action": "index" },

"request": { "REQUEST_PATH": "/test,, "REQUEST_METHOD": "GET" },

total time for request

rails controller/action

request env info

"mysql": { "queries": 3, "time": 0.00109302 },

"gc": { "calls": 8, "time": 2.04925 },

config.middleware.use(Memprof::Tracer)

8 calls to GC2 secs spent in GC

3 mysql queries

"objects": { "created": 3911103, "types": { "none": 1168831, "object": 1127, "float": 627, "string": 1334637, "array": 609313, "hash": 3676, "match": 70211 } }}

config.middleware.use(Memprof::Tracer)

3 million objs created

lots of strings

lots of arrays

regexp matches

object instances

1 million method calls

smiley-faces.org

mindfulsecurity.com

evil liveshttp://github.com/ice799/memprof/tree/dnw

• makes ruby faster!11!!1

• hooks read syscall

• looks for magic cookie (JOE)

• turns off GC

• Ruby is fast.

it makes ruby faster!!1!

look a bullshit benchmark!

it makes ruby faster!!1!#NORMAL RUBY!!!!11!!

[joe@mawu:/Users/joe/code/defcon/memprof/ext]% ab -c 10 -n 200 http://blah:4567/hi/JOE

Benchmarking blah (be patient)Completed 100 requestsCompleted 200 requestsFinished 200 requests

Concurrency Level: 10Time taken for tests: 7.462 secondsComplete requests: 200Failed requests: 0Write errors: 0Requests per second: 26.80 [#/sec] (mean)Time per request: 373.108 [ms] (mean)Time per request: 37.311 [ms] (mean, across all concurrent requests)

it makes ruby faster!!1!# fast0r RUBY!!!11!111[joe@mawu:/Users/joe/code/defcon]% ab -c 10 -n 200 http://blah:4567/hi/JOE

Benchmarking blah (be patient)Completed 100 requestsCompleted 200 requestsFinished 200 requests

Concurrency Level: 10Time taken for tests: 6.594 secondsComplete requests: 200Failed requests: 0Write errors: 0Requests per second: 30.33 [#/sec] (mean)Time per request: 329.708 [ms] (mean)Time per request: 32.971 [ms] (mean, across all concurrent requests)

you can do anything

• this example is stupid, but you can do anything.

• hook read/write and phone home with data.

• fork a backdoor when a specific cookie is seen

• whatever

break.com

zanyvideos.com

injectso

• written by Shaun Clowes

• injects libraries into running processes using ptrace(2).

• super clever hack!

hockeydrunk.com

injecting live processes

• ptrace(2)

• allows you to view and modify the register set and address space of another process

• permissions on memory are ignored

fucking injectso, how does it work?

• attach to target process using ptrace

• save a copy of a small piece of the program stack.

• save a copy of the register set

• create a fake stack frame with a saved return address of 0

fucking injectso, how does it work?• set register set to point at dlopen

• rip = &dlopen

• rdi = dso name

• rsi = mode

• let er rip, waitpid and it’ll segfault on return to 0.

• restore stack, register set, resume as normal.

ptrace evil dso• remote allocating

memory is a pain in the ass.

• generating segfaults in running processes might be bad (core dumps, etc).

• binary patching is hard, doing it with ptrace is harder.

• getting the user to use your library might be hard.

• already running processes will need to be killed first.

• need to poison each time app is started.

• binary patching is hard.

realmofraven.com

combine ‘em

• use injectso hack to load an evil dso

• evil dso will take it from there

64bit injectso port• ported by Stealth

• http://c-skills.blogspot.com/2007/05/injectso.html

• i did some trivial cleanup and put the codez on github

• http://github.com/ice799/injectso64

• tested it on 64bit ubuntu VM, works.

injectso +

evil-binary-patching-dso

customdynamics.com

customdynamics.com

buycostumes.com

emeraldinsight.com

how to defend against it• NX bit - call mprotect

• strip debug information - mostly prebuilt binaries

• statically link everything - extremely large binaries

• put all .text code in ROM - maybe?

• don’t load DSOs at runtime - no plugins, though

• disable ptrace - no gdb/strace.

• check /proc/<pid>/maps - word.

slashgear.com

my future research: exploring alternative

binary formats.

slayerinc.com

globalhealthandfitness.com

alignment

thomasgroup.com

calling convention

arianlim.wordpress.com

object file and library formats

tandemfs.org

questions?joe damato

@joedamatotimetobleed.com

http://timetobleed.com/string-together-global-offset-tables-to-build-a-ruby-memory-profiler/http://timetobleed.com/hot-patching-inlined-functions-with-x86_64-asm-metaprogramming/http://timetobleed.com/rewrite-your-ruby-vm-at-runtime-to-hot-patch-useful-features/http://timetobleed.com/dynamic-linking-elf-vs-mach-o/http://timetobleed.com/dynamic-symbol-table-duel-elf-vs-mach-o-round-2/

tallteacher.files.wordpress.com

“Interesting Behavior of OS X”

• Steven Edwards ([email protected])

• november 29 2007

• http://www.winehq.org/pipermail/wine-devel/2007-November/060846.html

leopard has a pe loader?

handle = dlopen("./procexp.exe", RTLD_NOW | RTLD_FIRST );

steven-edwardss-imac:temp sedwards$ ./a.outdlopen(./procexp.exe, 258): Library not loaded: WS2_32.dll Referenced from: /Users/sedwards/Library/ApplicationSupport/CrossOver/Bottles/winetest/drive_c/windows/temp/procexp.exe Reason: image not found

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